Rocks in a hydrocarbon reservoir store hydrocarbons (for example, petroleum, oil, gas, or combinations of one or more of them), for example, by trapping the hydrocarbons within porous formations in the rocks. Understanding properties of the hydrocarbon reservoir can assist to optimize extraction of the stored hydrocarbons from the reservoir. One technique to understand properties of the hydrocarbon reservoir is to develop computer-generated software models of all or portions of the reservoir. To develop such models, a reservoir rock sample from the hydrocarbon reservoir is evaluated and results of the evaluation are provided as an input to the computer software program that generates the software models. The reservoir rock sample can be evaluated by performing one or more of several experiments under laboratory conditions or under reservoir conditions (that is, the conditions experienced by the sample in the hydrocarbon reservoir). Rock wettability, specifically, the wettability of the porous structure within the rock, is one of the parameters of the reservoir rock sample that can be evaluated.